1. Field of the Invention
This invention relates generally to Schmitt triggers and, more particularly, to an MOS Schmitt trigger circuit exhibiting a constant difference between the positive and negative switching voltages.
2. Description of the Prior Art
The advantages offered by MOS technology are well known; e.g. higher density, greater yield, etc. Thus, smaller MOS device geometries permit a greater number of devices to be produced per unit area or, stated another way, a single MOS device will occupy less space. This characteristic is extremely important in the design and fabrication of complex digital integrated circuits; for example, single chip microprocessors.
Whereas digital circuitry is generally characterized by its "ON/OFF" or "ONE/ZERO" nature, most measurements in the real world are inherently analog; e.g., temperature, pressure, speed, voltage, etc. Therefore, it is necessary that microprocessors and other digital circuitry communicate or interface with analog circuitry such as amplifiers, buffers, comparators, etc., in order to permit digital processing of the analog signals. The required interfacing may be accomplished by providing analog components which are external to the microprocessor chip. However, such arrangements generally require more current, a larger power supply and commonly present more opportunities for design and manufacturing errors. To avoid these disadvantages, analog circuits are being manufactured integrally with the digital circuitry; e.g., on the microprocessor chip itself, and due to the complex nature of microprocessors, the inclusion of analog devices on the same chip requires that the same manufacturing process be employed.
Schmitt triggers are a commonly found interface between the analog and digital domains. To eliminate noise and spurious oscillations, the circuit is designed to have hysteresis between the upper and lower threshold values, which values are normally defined by two switching voltages V.sub.H and V.sub.L.
A disadvantage suffered by the standard MOS circuit resides in the inability to maintain the switching voltages V.sub.H and V.sub.L constant due to processing variations. Furthermore, as a result of said processing variations, as for example between the threshold voltages ofenhancement and depletion devices, the difference between V.sub.H and V.sub.L cannot be maintained constant when employing the standard circuit. Thus, the standard circuit could not be used to accurately control the frequency of, for example, a relaxation oscillator.